In a mobile communication system employing an LTE (Long Term Evolution)-Advanced scheme which is a next-generation communication scheme of an LTE scheme, it is possible to use a technology called “Carrier Aggregation” in which a mobile station UE communicates with a radio base station eNB by simultaneously using a plurality of frequency carriers.
The frequency carriers constituting the “Carrier Aggregation” are called “Component Carriers”.
Each “Component Carrier” may also serve as an independent LTE cell. That is, it may be possible to perform the “Carrier Aggregation” employing the LTE cell, which uses different frequency carriers, as “Component Carriers”.
Consequently, in the LTE-Advanced scheme, based on the “Carrier Aggregation”, it is possible to perform communication by simultaneously using a plurality of LTE cells using different frequency carriers.
Here, in the LTE scheme, a key KeNB used in a radio security process depends on both PCI (Physical Cell Identity, a physical cell ID) and “E-ARFCN (E-UTRA Absolute Radio Frequency Channel Number, a frequency code)” of a cell in communication.
This is for using a key generation function KDF (Key Derivation Function) using the PCI and the E-ARFCN as parameters when generating the key KeNB.
Furthermore, the key KeNB is generated from a master key KASME different for each mobile station UE. Therefore, in the LTE scheme, the key KeNB is a cell specific key and a mobile station UE specific key.